Method for producing a bar-type conductor

ABSTRACT

The invention relates to a process for manufacturing a conductor bar comprising a number of partial conductors, with the following steps:  
     A: manufacturing the partial conductors,  
     B: applying at least one insulating layer to the partial conductors, in each case with the following substeps:  
     B1: coating the regions to be insulated of the partial conductor with a plastics powder,  
     B2: sintering this powder layer by melting the plastic and at least partially curing the plastic to form an insulating layer, the sintering being carried out in such a way that at least one outer insulating layer is not completely cured,  
     C: assembling a number of insulated partial conductors to form a conductor bar,  
     D: sintering the insulating layers of the partial conductors while simultaneously pressing the partial conductors against one another in the assembled conductor bar, the plastic in the case of the not completely cured insulating layers at least partially melting and being squeezed out between the partial conductors, and the plastic remaining between the partial conductors curing substantially completely.

TECHNICAL FIELD

[0001] The invention relates to a process for manufacturing anelectrically conducting conductor bar comprising a number of partialconductors electrically insulated from one another. Conductor bars ofthis type may be used for example as stator conductors in rotatingelectrical machines, for example generators, or in transformers.Conductor bars in which the partial conductors are twisted in a certainmanner with respect to one another are also referred to as “Roebelbars”. Unlike steel cables, which are constructed from a multiplicity ofsteel wires of round cross section or hexagonal cross section, thepartial conductors of such a conductor bar regularly have a rectangularcross section.

PRIOR ART

[0002] In order to reduce the electrical losses, for example in arotating electrical machine, the partial conductors of the statorconductors used therein are insulated from one another. Accordingly, aconductor bar is assembled from partial conductors which are providedwith an insulation. The application of an insulation to the partialconductors may be carried out for example by a two-stage coating processin which, in a first stage, a plastics powder is applied to the surfaceof the partial conductor in order to form a powder layer. In a secondstage, this powder layer is sintered, the plastic melting and—dependingon the plastic—also crosslinking and curing. The insulating layerforming in the process must form a closed and substantially homogeneousfilm in order to perform its insulating function.

[0003] In order that an electrically insulating, closed andsubstantially homogeneous insulating layer is produced when the powderlayer is sintered, the application thickness with which the powder layeris applied to the partial conductor must have a specific minimumthickness. Experience shows that the lower limit for an applicationthickness for which this is satisfied lies around 50 μm. Thin insulatinglayers are of advantage, however, since on the one hand thecurrent-carrying cross section can be increased while the overall crosssection of the conductor bar remains the same, whereby the performancecapability of the machine equipped with it increases. On the other hand,the reduction in the insulating thickness of the partial conductors mayalso be used for a reduction in the overall cross section of theconductor bar, without thereby impairing the performance of the machineequipped with it. It can also be regarded as a further advantage of areduced partial conductor insulation that the heat transfer between thepartial conductors is improved as a result. This is important wheneverthe partial conductors of the conductor bar heat up differently onaccount of different losses. The improved heat transfer between thepartial conductors allows the different temperatures to be at leastpartially compensated again.

SUMMARY OF THE INVENTION

[0004] The present invention is concerned with the problem of providinga manufacturing process for a conductor bar constructed from a number ofpartial conductors which makes it possible for particularly thininsulating layers to be formed.

[0005] This problem is solved according to the invention by the featuresof claim 1.

[0006] The invention is based on the general idea of organizing thesintering operation in two stages in which, in a first sintering stage,at least part of the insulation of the individual partial conductors isnot completely cured and, in a second sintering stage, the partialconductors in the assembled conductor bar are pressed against oneanother, whereby part of the plastic forming the insulation is displacedand swells out between the partial conductors and only the plasticremaining in the insulation cures. Very small insulating thicknesses canbe achieved by this procedure. For example, the component parts of theinsulations which have already completely cured during the firstsintering stage, when the partial conductors are sintered, have theeffect during the second sintering stage, when the assembled conductorbar is sintered, that a sufficient insulating layer remains behindbetween the neighboring partial conductors to ensure the insulatingfunction of the insulation.

[0007] In the case of a preferred embodiment, a number of insulatinglayers are applied to the partial conductors, with at least theinnermost insulating layer being cured substantially completely. Thisensures that at least the completely cured insulating layer remainsbetween the partial conductors when the insulation is squeezed out andensures the insulating effect.

[0008] According to a particularly advantageous development, the atleast one, substantially completely cured, insulating layer may bederived from a powder layer of an application thickness which is lessthan approximately 50 μm. This embodiment makes use of the realizationthat, in a process according to the invention for forming an effectiveinsulation, it is not necessary for the at least one, substantiallycompletely cured, insulating layer of the respective partial conductorto form a closed and homogeneous film already during the manufacture ofthe partial conductor. This is so because, should gaps or holes exist inthis cured insulating layer, they are filled by a subsequently appliedinsulating layer, these fillings also remaining between the partialconductors when the insulation is squeezed out in the second sinteringstage. Accordingly, the desired closed homogeneous film for theinsulating layer can form at the latest in the assembled state of theconductor bar.

[0009] In the case of another embodiment, at least one insulating layerof the partial conductors which is not completely cured during thesintering of the partial conductors may be derived from a powder layerwhich is formed by a plastics powder with which there has been admixed acoarse filler, the average grain size of which is less than theapplication thickness of the powder layer. When the insulation issqueezed out, this coarse filler acts as a spacer and prevents theplastic from being squeezed out completely during the second sinteringstage and, as a result, ensures that at least enough plastic to form afully functional insulation between the partial conductors remainsbetween the partial conductors. This particular embodiment functionseven when none of the insulating layers of the partial conductors hasbeen completely cured during the first sintering stage, with the resultthat particularly thin insulating layers are achievable.

[0010] The process according to the invention operates particularlyexpediently whenever the plastic for forming the insulation is chosensuch that and the sintering of the second sintering stage is carried outin such a way that the partial conductors of the assembled conductor baradhesively bond to one another during the second sintering stage, whenthe insulation is cured. There is consequently no need for additionalfeatures for strengthening and stabilizing the assembled conductor bar.

[0011] Further important features and advantages of the processaccording to the invention emerge from the subclaims and from thedescription which follows of preferred exemplary embodiments of theinvention.

WAYS OF IMPLEMENTING THE INVENTION

[0012] In order to manufacture a conductor bar in a way corresponding tothe process according to the invention, firstly partial conductors whichare to be assembled into a conductor bar must be manufactured. Anuninsulated endless wire which has the cross section of the partialconductors to be manufactured and consists of an electrically conductingmaterial, for example copper, is used for the manufacture of thesepartial conductors. Undeformed partial conductors are then produced fromthis endless wire by cutting the endless wire to length, to the lengthof the partial conductors. Depending on the type of conductor bar to bemanufactured, the partial conductors may be deformed by bending, inparticular bent with an offset, after cutting to length, in order toachieve a desired three-dimensional form for the partial conductors.

[0013] At least one insulating layer is then applied to the partialconductors manufactured in this way. The application of such aninsulating layer is in this case carried out by a two-stage coatingprocess. In a first substep, the regions to be insulated of therespective partial conductor are coated with a plastics powder. Suitablein particular as the coating powder are thermosetting or elastomericplastics, such as for example epoxy and silicone resins. In order toimprove the mechanical properties of the insulation, for example inorder to reduce the coefficient of thermal expansion or reduce theflowing of the plastic at high temperatures and under high loads,inorganic fillers may be admixed with these plastics, for example inconcentrations of 10% to 80%.

[0014] The coating of the partial conductor is performed for example inan electrostatic fluidized-bed chamber or electrostatic spray chamber,which may be smaller than the partial conductor to be coated. Such acoating chamber has at opposite ends openings through which the partialconductors to be coated are drawn through the chamber individually ormore than one at a time. The application rate of the plastics powder,and consequently the application thickness of the powder layer, arecontrollable for example by means of the feed with which the partialconductor or the partial conductors are drawn through the chamber. Inorder that the plastics powder remains adhering on the surface of thepartial conductors, it is electrically charged for the coatingoperation.

[0015] This is then followed by the second substep, in which the appliedpowder layer is sintered. During the sintering, the plastic is heated,whereby it melts and at least partially cures, with the insulating layerbeing formed. If only a single insulating layer is applied to thepartial conductor, the sintering is controlled in such a way that thisinsulating layer does not cure completely.

[0016] If a number of insulating layers are applied to the partialconductor, the sintering is controlled in such a way that at least oneouter insulating layer is not completely cured.

[0017] This second substep is preferably performed in a separatesintering chamber and can similarly be carried out in a continuousprocess, it being possible for example for optical or electrical orinductive heating to be provided. During the sintering, the plasticspowder melts or fuses. In the case of thermosetting materials, there isalso thermal crosslinking of the plastic. The fusing and/or crosslinkinghas the effect that the desired insulating layer is formed. In the caseof thermosetting plastics, the degree of curing correlates with thedegree of crosslinking. Accordingly, in the case of thermosettingmaterials, the sintering process at least for one insulating layer iscontrolled in such a way that this thermosetting insulating layer is notcrosslinked or is crosslinked only partially. The degree of crosslinkingor the degree of curing may be controlled for example by means of thetemperature and by means of the duration of the thermal treatment. Thenot completely cured or crosslinked insulating layer produced in thisway is thermally still reactive.

[0018] Once the partial conductors have been provided in this way withone or more insulating layers, they are assembled to form a conductorbar. The conductor bar assembled in this way can then be bent into thedesired form that it is intended to have after its completion. In asuitable apparatus, the partial conductors are then pressed against oneanother in the assembled conductor bar. Carried out at the same time isa renewed sintering operation, in which the plastic of the notcompletely cured or crosslinked insulating layers at least partiallymelts again. The pressing to which the partial conductors are subjectedhas the effect that the melted plastic is squeezed out, with thethickness of the insulation between the partial conductors decreasing.The plastic squeezed out in this way collects for example incorresponding cavities of the conductor bar or on the surface of thebar.

[0019] During this operation referred to as “hot pressing”, theinsulating layers of the partial conductors adjacent to one another areadvantageously joined together, whereby the partial conductors of theassembled conductor bar adhesively bond to one another. After curing ofthe plastic, the conductor bar is made to set in the desired form.

[0020] In the case of a special exemplary embodiment, an epoxy resinpowder, which has for example an average grain size of about 40 μm, maybe used for forming the insulation. This epoxy resin powder is filledfor example with 40% TiO₂, this filler having for example a d50 value ofabout 0.2 μm. Four insulating layers are advantageously applied, withthe result that a thickness of about 100 μm is obtained for theinsulation after the sintering. The operation of sintering theinsulating layer first applied is controlled in this case in such a waythat the insulating layer cures almost completely, which corresponds forexample to a sintering duration of approximately three to five minutesat approximately 180° C. The three subsequent powder layers are sinteredonly to the extent that the minimum viscosity is just exceeded slightly;this corresponds to a sintering duration of approximately ten to twentyseconds. As a result, the three last insulating layers are not cured, orcured only slightly, with the result that the plastic remainssubstantially in the so-called “B stage”. Accordingly, of the entireinsulation, approximately three quarters are in the B stage. During hotpressing of the assembled conductor bar, the plastic in the B stagebecomes free-flowing and is squeezed out from the composite assembly. Inthe case of this exemplary embodiment, the thickness of the insulationcan be reduced during hot pressing of the conductor bar to approximately30 μm, without the quality suffering with regard to homogeneity and theinsulating effect of the insulation thereby produced.

[0021] In comparison with a conventional procedure, in which theapplication thickness of the powder layer must comprise approximately 50μm in order to produce a homogeneous layer, the insulating layerthickness can consequently be approximately halved by the processaccording to the invention.

[0022] In the case of another special embodiment, an epoxy resin powderwhich has an average grain size of approximately 40 μm is used forproducing the insulation. The epoxy resin powder is filled for examplewith 30% TiO₂, it again being possible for the average grain size ofthis filler to be approximately 0.2 μm. In a way corresponding to thisspecial embodiment, a coarse filler is also admixed with the plasticspowder. This coarse filler may form for example approximately 10% of theproportion by mass of the plastics powder. The coarse filler preferablycomprises an electrically insulating material, for example ceramic orquartz. Quartz glass beads which have for example an average grain sizeof approximately 15 μm are preferred here. This plastics powder is alsoapplied for example in four layers, with the result that the overallthickness of the sintered insulation is also about 100 μm. Here, too,each individual insulating layer is thermally sintered, the sinteringprocess, in particular the duration of the heat treatment, for eachlayer being controlled in such a way that no curing takes place, orminimal curing in which the minimum viscosity is just exceeded slightly;the sintering of each layer at approximately 180° C. takes for exampleonly ten to twenty seconds. This procedure has the effect that all theinsulating layers remain substantially in the non-cured B stage.

[0023] During the subsequent operation of hot pressing the assembledconductor bar, the coarse fillers serve as spacers and have the effectthat the liquefied plastic is not completely squeezed out. The coarsefiller also ensures that neighboring partial conductors do not come intoelectrical contact with one another during the hot pressing.

[0024] In the case of this embodiment, a final thickness for theinsulation between two neighboring partial conductors of approximately20 μm can be achieved during the hot pressing, no compromises withregard to the homogeneity of the achievable insulation having to be madehere either. Since the insulating layer formed in this way is in eachcase to be attributed to both partial conductors between which it isformed, each individual partial conductor has an (imaginary) insulatinglayer with a thickness of approximately 10 μm. By comparison with aconventionally produced layer thickness, this provides an improvement bya factor of 5.

1. A process for manufacturing an electrically conducting conductor barcomprising a number of partial conductors electrically insulated fromone another, with the following steps: A: manufacturing the partialconductors, B: applying at least one insulating layer to the partialconductors, in each case with the following substeps: B1: coating theregions to be insulated of the partial conductor with a plastics powder,B2: sintering this powder layer by melting the plastic and at leastpartially curing the plastic to form an insulating layer, the sinteringbeing carried out in such a way that at least one outer insulating layeris not completely cured, C: assembling a number of insulated partialconductors to form a conductor bar, D: sintering the insulating layersof the partial conductors while simultaneously pressing the partialconductors against one another in the assembled conductor bar, theplastic in the case of the not completely cured insulating layers atleast partially melting again and being squeezed out between the partialconductors, and the plastic remaining between the partial conductorscuring substantially completely.
 2. The process as claimed in claim 1,characterized in that in step B a number of insulating layers areapplied, with at least the innermost insulating layer being curedsubstantially completely.
 3. The process as claimed in claim 2,characterized in that the at least one substantially completely curedinsulating layer is derived from a powder layer of an applicationthickness which is less than approximately 50 μm.
 4. The process asclaimed in claim 2 or 3, characterized in that only the innermostinsulating layer is cured substantially completely and in that all theother insulating layers are not completely cured.
 5. The process asclaimed in one of claims 1 to 4, characterized in that at least oneinsulating layer which is not completely cured in step B is derived froma powder layer which is formed by a plastics powder with which there hasbeen admixed a coarse filler, the average grain size of which is lessthan the application thickness of the powder layer.
 6. The process asclaimed in claim 5, characterized in that in step B all the insulatinglayers are not completely cured.
 7. The process as claimed in claim 5 or6, characterized in that the coarse filler consists of an electricallyinsulating material.
 8. The process as claimed in one of claims 1 to 7,characterized in that the plastic for forming the insulation is chosensuch that the sintering according to step D is carried out in such a waythat the partial conductors of the assembled conductor bar adhesivelybond to one another.